(a) Field of the Invention
The present invention relates to an electrolyte comprising a non-ionic surfactant and to a lithium ion battery using the same, and more particularly, to a non-aqueous electrolyte for a lithium ion battery comprising a fluorine-based non-ionic surfactant.
(b) Description of the Related Art
Ever since the commercialization of lithium ion liquid secondary batteries by Sony Co., the lithium ion liquid secondary battery has been used increasingly in portable computers, cellular phones, etc., instead of the prior art lithium ion secondary batteries as a result of its high energy density. The lithium ion liquid secondary battery comprises an anode including carbonaceous material as an anode active material and a cathode including metal oxide of lithium cobalt oxide (LiCoO2), etc., as a cathode active material, and is prepared by intercalating a porous polyolefin-based separator between the anode and the cathode, then by injecting a non-aqueous electrolyte having a lithium salt of lithium hexafluorophosphate (LiPF6), etc. When the battery charges, the lithium ions of the cathode active material are released and then inserted into the carbon layer of the anode. When the battery discharges, the opposite occurs with the lithium ions of a carbon layer of an anode being released and then inserted into the cathode active material.
The non-aqueous electrolyte plays a mediating role moving the lithium ions between the anode and the cathode. The electrolyte should be stable within the scope of the operation voltage of the battery, and be able to transfer the ions sufficiently at a fast velocity. As an electrolyte, U.S. Pat. Nos. 5,521,027 and 5,525,443 discloses an admixture electrolyte of a linear carbonate and cyclic carbonate. The cyclic carbonate has a large polarity and thus is sufficiently capable of dissociating lithium, but has high viscosity. Therefore, in these patents, mixing linear carbonate with a low polarity and a low viscosity reduces the viscosity of the electrolyte comprising the cyclic carbonate. The different linear carbonates include dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), etc. The different cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), vinylene carbonate (VC), butylene carbonate (BC), etc. The use of cyclic carbonates is necessary when it is desired to obtain a light capacity, good temperature properties, and a safe battery configuration, and particularly, when using electrolytes having a high viscosity such as EC, PC, etc.
The non-aqueous electrolyte decreases operational efficiency because it is slowly penetrated into the active material of electrodes, and thus the performance of the battery deteriorates by increasing an impedance thereof because a sufficient capacity of the battery cannot be utilized. Accordingly, in order to improve an interfacial property between the non-aqueous electrolyte and electrodes, Japanese Patent Publication Hei 8-306386 discloses a method of adding an anionic surfactant to an electrolyte and Japanese Patent Publication Hei 9-30651 discloses a method of adding an anionic surfactant directly to the electrode slurry. However, satisfactory results have not been obtained with these two methods. The surfactant that is contained in an electrolyte should not affect the other properties of a battery, should be stable in the operation voltage range of a battery and increase an interfacial activity between electrodes and an electrolyte.